The prostate-specific membrane antigen (PSMA) is increasingly recognized as a viable target for imaging and therapy of prostate and other forms of cancer (Ghosh and Heston, 2004; Milowsky et al., 2007; Olson et al., 2007). PSMA is significantly over-expressed in PCa and metastases, particularly with respect to the hormone-refractory form (Ghosh and Heston, 2004; Milowsky et al., 2007). PSMA also is known to express by most solid tumors and tumor neovasculature (Haffner et al., 2012; Haffner et al., 2009). Imaging PSMA can provide insight into androgene signaling (Evans et al., 2011) and response to taxane therapy (Hillier et al., 2011). Previous studies have demonstrated PSMA-targeted radionuclide imaging in experimental models of prostate cancer (Schulke et al., 2003; Mease et al., 2013; Banerjee et al., 2010) and in the clinic (Cho et al., 2012; Kulkarni et al., 2014; Zechmann et al., 2014) using functionalized cysteine-glutamate or lysine-glutamate ureas. For the attachment of large molecular fragments, such as radiometal (99mTc, 68Ga, 111In, 86Y, 203Pb, 64Cu) complexes (Banerjee, Pullambhatla, Shallal, et al., 2011; Banerjee, Pullambhatla, Byun, et al., 2011; Banerjee et al., 2008) and nanoparticles (Chandran et al., 2008; Kam et al., 2012), a long linker was placed between the large molecule and the targeting urea to retain PSMA-targeted binding. Without wishing to be bound to any one particular theory, it was thought that PSMA would be a suitable biomarker for MR molecular imaging because of the extra-cellular location of the ligand binding site and the estimated high receptor concentration per cell (˜3.2 μM/cell volume).
MR imaging is a clinically relevant, noninvasive diagnostic tool for providing high resolution anatomic and functional imaging. Molecular MR imaging enables the visualization of biological markers in vivo (Artemov, Mori, Okollie et al., 2003; Artemov, Mori, Ravi, Bhujwalla, et al., 2003; Konda et al., 2001; Lanza et al., 2004; Huang, et al., 2013). Gd(III)-based contrast agents are widely accepted by clinicians because they are easy to administer and provide T1-weighted, positive contrast. Although progress has been made in the design of contrast agents with high relaxivity, sensitivity remains a limiting factor for molecular MR imaging. For use in molecular imaging applications (specifically, for imaging receptors or protein expression), Gd(III)-based contrast agents seldom exceed the limit of detection (Artemov, Mori, Okollie et al., 2003; Artemov, Mori, Ravi, Bhujwalla, et al., 2003; Konda et al., 2001; Lanza et al., 2004; Huang, et al., 2013). With signal amplification strategies, MR might offer a sensitive modality for molecular imaging complementary to radionuclide-based techniques (Aime et al., 2004; Major et al., 2009; Song et al., 2008; Artemov, 2003). Although amplification strategies could improve the sensitivity of a targeted agent, shifting from a simple, low-molecular-weight compound to a larger, multiplexed entity may significantly alter the pharmacokinetic profile of the agent (Artemov, Mori, Okollie et al., 2003; Artemov, Mori, Ravi, Bhujwalla, et al., 2003; Konda et al., 2001; Lanza et al., 2004; Huang, et al., 2013). Sherry et al. have addressed the issue of sensitivity by generating contrast agents with very high binding affinities (Kd) such that the amount of agent needed for detection by MR could be minimized (Hanaoka et al., 2008; De Leon-Rodriguez et al., 2010). Combining a receptor-specific high affinity ligand together with multimeric Gd(III) agents for detection has been devised as one solution for enabling MR-based receptor imaging (Wu et al. 2012).
An example of that approach includes molecular imaging of VEGFR2 by preparing a multimeric Gd-dendron with high longitudinal relaxivity (r1) values (De Leon-Rodriguez et al., 2010). Other multimeric agents have been reported with improved r1 values at higher field strengths since MR imaging, both experimental and clinical, are moving to higher fields (Mastarone 2011). Optimizing relaxivity at high field provides the advantages of greater signal-to-noise and contrast to noise ratios (SNR/CNR) and the attendant benefits of higher spatial resolution and reduced acquisition times (Rooney 2007). Combination of these concepts, namely use of high-affinity targeting moieties with sensitive multimeric contrast agents, provides rationale to investigate targeted MR imaging of cells and tissues expressing the prostate-specific membrane antigen (PSMA).
Further, it has been reasoned that urea-based agents could also be used for radiotherapy of PSMA-containing lesions using radionuclides. In fact, clinical studies using that approach with [131I]MIP1095 ((S)-2-(3-((S)-1-carboxy-5-(3-(4-[131I]iodophenyl)ureido)pentyl)ureido)pentanedioicacid) (Zechmann et al., 2014) and 177Lu-labeled PSMA-targeted agents (Kulkarni et al., 2014) are under way for the treatment of castrate-resistant prostate cancer. This will be in analogy with radioimmunotherapy (RIT), which has proved remarkably successful in the treatment of lymphoma with two commercial products routinely integrated into clinical practice. However, RIT is fraught with difficulties due to the use of radiolabeled antibodies for imaging, including prolonged circulation times, unpredictable biological effects and the occasional need for pre-targeting strategies. Furthermore, antibodies may have less access to tumor than low molecular weight agents, which can be manipulated pharmacologically. Therefore a need remains for low molecular weight compounds with high binding affinity to PSMA for the imaging and radiotherapy of tumors.
The positron-emitting radionuclide 86Y (half-life [t1/2]=14.74 h, β+=33%, Eβ+=664 keV) is an attractive isotope for molecular imaging (Nayak and Brechbiel, 2011). Yttrium-86 can readily be prepared on a small biomedical cyclotron employing the 86Sr(p, n)86Y nuclear reaction (Yoo et al., 2005). The extensive use of the high-energy β−-emitter 90Y (t1/2=64.06 h, β−=72%, Eβ−=2.288 MeV) for endoradiotherapy (Witzig et al., 2003; Bodei et al., 2004) makes 86Y ideal for dosimetry estimates of 90Y-labeled radiotherapeutics (Helisch et al., 2004). Antibodies and peptides radiolabeled with 86Y have identical properties to those labeled with 90Y, enabling accurate absorbed dose estimates for 90Y for radiotherapeutics (Nayak and Brechbiel, 2011; Palm et al., 2003). Although 177Lu has a shorter β-particle range (t1/2=6.7 days, Eβ−=0.5 MeV) than 90Y, is because they have similar chelation chemistry, 86Y proposed as a suitable imaging surrogate to investigate potential 177Lu-based radiotherapeutics, as well as those radiolabeled with 90Y. A similar rationale has been applied to agents for neuroendocrine-targeted peptide receptor radionuclide therapy (Chen et al., 2012). Using similar approach, a potential matched-pair imaging radioisotope 203Pb (half-life, 51.9 h, Eβ−=279-keV γ-ray, 81%) suitable for SPECT imaging can be used for therapeutic radionuclide 212Pb for α-particle therapy (Chappell, et al. 2000; Yong, et al. 2011; Yong, et al. 2012; Yong, et al. 2013). The decay scheme of 212Pb includes 212Bi, which yields an α-particle, two β-particles, and several γ-emissions upon decay. α-Particle emitters are particularly attractive for targeted radiotherapy due to high linear energy transfer properties such as localized dense ionization, which results in irreparable DNA double-strand breaks and cytotoxicity that is independent of tissue oxygen content or dose rate (McDevitt, et al., 1998). 212Pb and 212Bi are both promising α-particle emitting sources that have well-described radiochemistry for antibody linkage and are readily obtained from a 224Ra generator.
Radiohalogenated carbamate based PSMA inhibitors that also demonstrated high binding affinity to PSMA in-vitro also have been developed and when radiolabeled with the positron emitter F-18 showed high uptake in PSMA positive mouse tumor xenografts with fast clearance from normal tissues. Because of the favorable pharmacokinetic profile of this class of compounds, i.e., low nonspecific binding, lack of metabolism in vivo and reasonable tumor residence times, the imaging studies have been extended to molecular radiotherapy. Moreover, carbamate-based inhibitor can be coupled to metal-chelating agent employing a linker functionality similar as urea-based metal/radiometal-based agents to maintain high binding affinity for PSMA. Consequently, metal or radiometal comjugated carbamate scaffold can also be utilized for imaging and therapy of PSMA-expressing cells and tissues.